One of the fastest growing worldwide markets in the metal finishing/electroplating industry is the processing and plating of aluminum and its alloys. Aluminum's unique physical and mechanical characteristics make it particularly attractive for industries such as automotive, electronics, telecommunications and avionics, along with a plethora of decorative applications. Among aluminum's most endearing properties include it's low overall density (2.7 g/cc), high mechanical strength achieved through alloying and heat treating, and its relatively high corrosion resistance. Additional properties of aluminum include; high thermal and electrical conductance, its magnetic neutrality, high scrap value, and its amphoteric chemical nature. Aluminum articles for many applications are made from a variety of aluminum alloys with alloying elements including: silicon, magnesium, copper, etc. These alloying mixes are formed in order to achieve enhanced properties such as high-strength or ductility.
The plating of aluminum and its alloys requires specific surface preparations for successful electrolytic and electroless deposition. The most common practice used in order to achieve successful electrodeposition is applying an immersion zinc coating (better known as zincate) to the substrate just prior to plating. This procedure has long been considered the most economical and practical method of pre-treating aluminum. The major benefits of applying a zincate layer for pretreatment are the relative low cost of equipment and chemistry, wider operating windows for processing, and ease of applying a controlled deposit.
The presence of other metals in the zincate solutions has an affect on the rate and efficacy of the zinc deposition. Small amounts of alloy components (i.e. Fe, Ni, Cu) improve not only the adhesion of the zincate deposit, but also increase the usability of the zincate on a variety of aluminum alloys. For example, the addition of Fe ions improves the adhesion on magnesium containing alloys. The presence of nickel in the zincate improves the adhesion of nickel plated directly onto the zincate, and similar effects can be found with addition of copper in the zincate and subsequent copper plate. In general, however, the alloying of zincate has shown to provide thinner and more compact deposits which effectively translate into better adhesion of downstream electroless/electrolytic plating. On the other hand, the composition of an alloying zincate becomes more and more complicated with the additional metal ions in the composition. It makes selection of complexing agents more complicated and critical for the overall performance of the zincate. A zinc-iron-nickel composition is more sensitive than zinc-iron compositions for the selection of complexing agents and ratio of metal ions in the composition. This becomes even more critical with the addition of the cooper ions in the alloy zincate. Due to its noble position in the galvanic series, the deposition rate of copper in the immersion zincate deposition is much higher than the other elements in the zincate. Therefore, control of the deposition rate of copper becomes important. It is possible to control the deposition rate of copper by the selection of the right complexing agent(s) for copper ions and adequate ratio with the other metal ions. There are few strong complexing agents for copper ions which offer good stability and performance of the alloying zincate, and cyanide appears to be the best candidate. Cyanide is a complexer of choice for the copper containing zincate compositions and it has been the industry standard for that application for many years. A negative aspect for the use of cyanide is the extremely toxic nature of cyanide, and therefore, like other metal finishing products, the search for a cyanide replacement in the alloying zincate has been a topic of interest for many years.
In recent years there are some non-cyanide alloy zincate compositions that have been developed but these compositions still contain hard complexing agents such as EDTA, NTA, ethylene diamine, etc. to keep the multi-ion system in the stable form which makes waste treatment of spent zincate solutions as well as its rinses more difficult. Zincate treatments also generally perform better under multiple treatment modes. Aluminum pretreatment with a single dip in the zincate doesn't produce as good results as the process with double or triple dips in the zincate prior to the subsequent plating step. Such multiple zincate process requires more processing steps and time, and means a more complicated and less productive and economical process. Therefore, like other metal finishing products, search for replacement of conventional alkaline cyanide or non-cyanide alloying zincate for the plating on aluminum has been a topic of interest in recent years.